Durable life of concrete structures under the chloride environment depends on the permeability of chloride ion, and the chloride ion diffusion coefficient is the main indicator that reflects the permeability of chloride ion. Based on the Fick’s second law, a multi-factor model of the chloride ion diffusion is established. In this model, the influences of temperature, humidity, age of the concrete, fly ash and carbonation are taken into consideration. And the model is verified by engineering test data from the concrete structure under the wet and dry areas of marine. The results show that the calculated and measured values of the chloride ion content agree well, the actual situation of the project could be basically reflected. The multi-factor model of the chloride ion diffusion is practical.